This invention relates to an improved method of formation flow testing for oil and gas wells. This invention is particularly useful in the testing of offshore wells where it is desirable to conduct testing operations and well stimulation operations utilizing the testing string tools with a minimum of testing string manipulation, and preferably with the blowout preventers closed during most operations.
It is known in the art that tester valves and sampler valves for use in oil and gas wells may be operated by applying pressure increases to the fluid in the annulus between the wellbore and testing string therein of a well. For instance U.S. Pat. No. 3,664,415 to Wray et al discloses a sampler valve which is operated by applying annulus pressure increases against a piston in opposition to a predetermined charge of inert gas. When the annulus pressure overcomes the gas pressure, the piston moves to open a sampler valve thereby allowing formation fluid to flow into a sample chamber contained within the tool, and into the testing string facilitating production measurements and testing.
In U.S. Pat. No. 3,858,649 to Holden et al a tester valve is described which is opened and closed by applying pressure changes to the fluid in the annulus contained between the wellbore and testing string therein of a well. The tester valve contains a supplementing means wherein the inert gas pressure is supplemented by the hydrostatic pressure of the fluid in the annulus contained between the wellbore and testing string therein as the testing string is lowered into the well. This feature allows the use of lower inert gas pressure at the surface and provides that the gas pressure will automatically be adjusted in accordance with the hydrostatic pressure and environment at the testing depth, thereby avoiding complicated gas pressure calculations required by earlier devices for proper operation. The tester valve described in U.S. Pat. No. 3,856,085 to Holden et al likewise provides a supplementing means for the inert gas pressure in a full opening testing apparatus.
This supplementing means includes a floating piston exposed on one side to the inert gas pressure and on the other side to the annulus fluid pressure in order that the annulus fluid pressure can act on the inert gas pressure. The system is balanced to hold the valve in its normal position until the testing depth is reached. Upon reaching the testing depth, the floating piston is isolated from the annulus fluid pressure so that subsequent changes in the annulus pressure will operate the particular valve concerned.
This method of isolating the floating piston has been to close the flow channel from the annulus contained between the wellbore and testing string in a well to the floating piston with a valve which closes upon the addition of weight to the testing string. This is done by setting the testing string down on a packer which supports the testing string and isolates the formation in the well which is to be tested during the test. The apparatus, which is utilized to isolate the floating piston is designed to prevent the isolation valve from closing prematurely due to increasingly higher pressures as the testing string is lowered into the well, contains means to transmit the motion necessary to actuate the packer and is designed to remain open until sufficient weight is set down on the packer to prevent premature isolation of the gas pressure and thus premature operation of the tester valve.
However, since the tester valve described in U.S. Pat. No. 3,856,085 contains a weight operated isolation valve, the tester valve may inadvertently open when being run into the well on a testing string, if a bridge is encountered in the wellbore thereby allowing the weight of the testing string to be supported by the tester valve. Also, in this connection, in highly deviated wellbores it may not be possible to apply sufficient weight to the testing string to actuate the isolation valve portion of the tester valve thereby causing the tester valve to be inoperable. Furthermore, if it is desired to utilize a slip joint in the testing string, unless weight is constantly applied to the slip joint to collapse the same, the isolation valve portion of the tester valve will open thereby causing the tester valve to close.
In U.S. Pat. No. 3,976,136 to Farley et al a tester valve is described which is opened and closed by applying pressure changes to the fluid in the annulus contained between the wellbore and testing string therein of a well and which contains a supplementing means wherein the inert gas pressure is supplemented by the hydrostatic pressure of the fluid in the annulus contained between the wellbore and testing string therein as the testing string is lowered into the well. This tester valve utilizes a method for isolating the gas pressure from the annulus fluid pressure which is responsive to an increase in the annulus fluid pressure above a reference pressure wherein the operating force of the tool is supplied by the pressure of a gas in an inert gas chamber in the tool. The reference pressure used is the pressure which is present in the annulus at the time a wellbore sealing packet is set to isolate one portion of the wellbore from another.
The annulus fluid pressure is allowed to communicate with the interior bore of this tester valve as the testing string is lowered in the wellbore and is trapped as the reference pressure when the packer seals off the wellbore thereby isolating the formation in the well which is to be tested. Subsequent increases in the well annulus pressure above the reference pressure activates a pressure response valve to isolate the inert gas pressure from the well annulus fluid pressure. Additional pressure increases in the well annulus causes the tester valve to operate in the conventional manner.
Once a well has been tested to determine the contents of the various formations therein, it may be necessary to stimulate the various formations to increase their production of formation fluids. Common ways of stimulating formations involve pumping acid into the formations to increase the formation permeability or hydraulic fracturing of the formation to increase the permeability thereof or both.
After the testing of a well, in many instances, it is highly desirable to leave the testing string in place in the well and stimulate the various formations of the well by pumping acids and other fluids into the formations through the testing string to avoid unnecessary delay by pulling the testing string and substituting therefor a tubing string.
During well stimulation operations in locations during extremely cold environmental periods where the tester valves described in U.S. Pat. Nos. 3,856,085 and 3,976,136 are utilized in the testing string if large volumes of cold fluids are pumped through the tester valves, even though the formations surrounding the tester valves may have a temperature of several hundred degrees fahrenheit, the tester valves will be cooled to a temperature substantially lower than the surrounding formations by the cold fluids being pumped therethrough. When these tester valves are cooled by the cold fluids, the inert gas in the valves contracts. Upon the cessation of the pumping of cold fluids through the tester valve, if it is desired to close the tester valve by releasing the fluid pressure in the annulus between the wellbore and testing string, since the inert gas has contracted due to the cooling of the valve, the inert gas in its cooled state may not exert sufficient force to close the tester valve to thereby isolate the formation which has been stimulated from the remainder of the testing string.
The annulus pressure responsive tester valve disclosed in U.S. Pat. No. 4,422,506 includes a pressure assisted isolation valve which includes a pressure differential metering cartridge to control the rate at which the isolation valve returns to the fluid pressure in the annulus between the wellbore and testing string thereby continuously controlling the rate of expansion the inert gas within the gas chamber and the attendant operation of the tester valve regardless of any cooling effect by cold fluids pumped therethrough. The tester valve disclosed therein embodies improvements over the prior art valves described in U.S. Pat. Nos. 3,856,085 and 3,976,136 to eliminate undesirable operating characteristics thereof by including a pressure differential metering cartridge which is similar to that described in U.S. Pat. No. 4,113,012.
All of the above prior art devices, and their methods of use, entail running into the well with the tester and/or sampler valve (generally referred to as a tool bore closure valve) of the testing string in the closed position. This presents a disadvantage in that the testing string cannot automatically fill with well fluids as it is run into the well, which would save the well operator considerable rig time, whether a packer is included in the testing string or the testing string stings into a previously set production packer. In addition, the use of a tool bore closure valve which could be run into the well in an open position, and hence permit filling of the testing string, would prevent a pressure buildup between the tool bore closure valve and the valve in a production packer when the bottom of the testing string "stings" into a production packer set above a producing oil formation prior to opening the packer valve. Furthermore, it would be desirable to be able to pressure test a packer after setting the packer by pressuring up the annulus without cycling the tool bore closure valve, a feature which present tools do not offer. Finally, an initially open tool bore closure valve would permit the spotting of a water cushion or treating fluids into the testing string prior to running the test, by displacing well fluid out the bottom of the testing string, or setting the test string packer, if one is employed therewith.
Attempts have been made to provide an open tool bore closure valve when running into the wellbore, by reversing the normal mounting position of the ball valve used in prior art tester valves so that an increase, instead of a decrease, in annulus pressure closes the ball valve. Needless to say, this arrangement is extremely dangerous as the tool operator must maintain elevated annulus pressure continuously, or the tester valve will open and the upper testing string and surface equipment will be exposed to formation pressure.